DETECTOR DEVICE

Abstract: There is disclosed a detector device for detection of gas-borne contaminant having a sample gas flow passage and a window into said passage for ionising radiation and shield means interposing a flow of shielding gas between said window and said sample gas flow against contamination of said window.

Description: DETECTOR DEVICE f This invention relates to detector devices fo gas-borne contaminant. One such detector device is described i PCT/GB92/01313 and comprises a sample gas flow passag and a window into said passage for ionising radiation The ionised sample gas passes between measurin electrodes. Although the device is very sensitive t low contaminant concentrations, it is also very toleran of high concentrations. A problem arises, however, o contaminant fouling the window which reduces th sensitivity and can give rise to spurious results. The present invention alleviates that problem. The invention comprises a detector device fo detection of gas-borne contaminant having a sample ga flow passage and a window into said passage for ionisin radiation and shield means interposing a flow o shielding gas between said window and said sample ga f flow against contamination of said window. * ! The device may comprise an ionising radiatio source chamber behind said window and shielding gas flo means for shielding gas from said chamber over sai window into said passage. Said chamber may have filtered inlet. Sample gas flow past said window may draw sai shielding gas flow in train. Said shielding gas flow may comprise a thi layer. The source of ionising radiation may comprise a ultraviolet source. The device may comprise baffle means definin the sample and shielding gas flow paths and which ma comprise radiation-unaffected material such as poly tetrafluoroethylene, which is tolerant of ultraviole radiation and which does not, when subject to suc radiation, emit photoelectrons, which would interfer with measurements. The shielding gas may be clean (e.g. filtered air, but the device may comprise a source of shieldin gas such as a reservoir of clean air or nitrogen. One embodiment of a detector device according t the invention wjJLl now be described with reference t the accompanying drawings, in which the single Figure i a diagrammatic lengthwise section through a detecto device of the type disclosed in PCT/GB92/01313 afore mentioned. The detector device illustrated in the drawin comprises a sample gas flow passage 11 through whic sample gas, e.g. air which might carry contaminant, i drawn by a fan 12. The device has a window 13 openin into the passage 11 for ionising radiation which ionise the sample gas. The ionised sample gas then passes b an electrode system 14 which detects the ionisatio occasioned by the radiation. An ultraviolet source 15 is located behind th window 13 in a chamber 16. The source 15 has an en face comprising the window 13 which in use with heavil contaminated samples could itself become coated wit contaminant which will affect its performance - th radiation emitted through the window 13 will b attenuated, and the coating, itself excited by th radiation, could emit photoelectrons that woul interfere with measurement. Against such contamination, according to th invention, the window 13 is protected by a flow o shielding gas 1J . between the window 13 and the sampl gas flow through the passage 11. The chamber 16 is hermetically sealed except fo a filtered inlet 17 and a shielding gas outlet apertur 18a. Sample gas flow past the window 13 (pulled by th fan 12) draws the shielding gas flow in train as a thi layer flow over the window 13. Instead of relying on filtered air drawn throug the chamber 16 by the fan 12, a definitive source o shielding gas may be provided as a reservoir, such as pressure cylinder 19 connected through suitable valvin 21 to the chamber 16. The shielding gas may then b clean air or it may be nitrogen, which does not absor 10.6 eV UV radiation and does not, on that account affect the sensitivity of the device. In practice however, an air shield would reduce the sensitivity onl by about 20% which is a small price for the longer ter stability of the detector. Dilution of the sample gas by the shielding ga need cause a reduction in contaminant concentration o one or two percent at most, which will be of little o no consequence, but can be compensated for in any even during calibration of the device. The aperture 18a is in a baffle 22 defining th sample and shielding gas flow paths. The aperture 18 can be the same size as or slightly smaller than th window 13 - as it will be subject to some radiation, i is desirably of a material which is not affected. Poly tetrafluoroethylene is tolerant of UV radiation and doe not emit photoelectrons on exposure. The face of th baffle 22 in the passage 11 can be metallised an earthed to help define and maintain the electri potential distribution within the detection device. The gas shielding arrangement is of cours applicable to any window into the passage, not just window for ionising radiation. A detector device ma have several windows into the passage for one reason o another; each window can be shielded in similar fashion.

Claims: CLAIMS 1. A detector device for detection of gas-born contaminant having a sample gas flow passage and window into said passage for ionising radiation an shield means interposing a flow of shielding gas betwee said window and said sample gas flow agains contamination of said window. 2. A device according to claim 1, comprising a ionising radiation source chamber behind said window an shielding gas flow means for shielding gas from sai chamber over said window into said passage. 3. A device according to claim 2, said chambe having a filtered inlet. 4. A device according to any one of claims 1 to 3 in which sample gas flow past said window draws sai shielding gas flow in train. 5. A device according to any one of claims 1 to 4 in which said shielding gas flow comprises a thin layer. 6. A device according to any one of claims 1 to 5 in which the source of ionising radiation comprises a ultraviolet source. 7. A device according to any one of claims 1 to 6 comprising baffle means defining the sample an shielding gas flow paths. 8. A device according to claim 7, in which sai baffle means comprise radiation - unaffected materia such as polytetra-fluoroethylene. 9. A device according to any one of claims 1 to 8 in which the shielding gas is clean air. 10. A device according to any one of claims 1 to 8 comprising a source of shielding gas. 11. A device according to claim 10, in which sai source comprises a reservoir of clean air. 12. A device according to claim 10, in which sai source comprises a reservoir of nitrogen.

Inventors: JONES CHRISTOPHER D, LIGHTFOOT HOWARD WILLIAM, JONES, CHRISTOPHER, D., LIGHTFOOT, HOWARD, WILLIAM
Applicants: ENVIRO SYSTEMS LTD, JONES CHRISTOPHER D, LIGHTFOOT HOWARD WILLIAM, ENVIRO SYSTEMS LIMITED, JONES, CHRISTOPHER, D., LIGHTFOOT, HOWARD, WILLIAM
Application year: 1994
Main classification: G01N27
IPC codes: G01N27